1. Field of the Invention
This invention relates to contact lenses, and more particularly, to soft contact lenses of the toric type designed to correct astigmatism in humans.
2. Description of Prior Art
A toric lens has both spherical and cylinder correction and accordingly must maintain a fixed and predetermined orientation during use. A normal spherical lens tends to rotate in the eye as it floats on tears and is subject to normal blinking and eye movement. The problem of preventing rotation of a toric lens has been addressed in several ways.
A prism ballast has been used in soft contact lenses to provide orientation by gravitational forces, but the greater thickness of the lens over the area of maximum ballast, and particularly the thickness of the edge of the lens, has resulted in patient discomfort. To allievate this problem, the degree of prism has generally been limited to a maximum of from about 0.75 to 1.0 diopters. With lower prism values, the lens is often additionally truncated at the prism base since the ballast alone is insufficient to assure proper orientation. It has also been suggested to chamfer the edge of the lens over the area of the prism base to improve patient comfort. These modifications of the lens have not proved to be entirely satisfactory.
U.S. Pat. No. 4,095,878 discloses a method for assuring axial orientation in which a soft contact lens having a relatively thick edge is flattened along a portion of its peripheral region in the 12 o'clock and 6 o'clock positions so that the thickness of the edges in these regions is less than the thickness in the regions of 9 o'clock and 3 o'clock. Because the relatively thick edges remaining in the 9 o'clock and 3 o'clock positions may still cause discomfort to the wearer, it is suggested to taper off these regions to a limited degree. The technique proposed by this reference has not proved to be entirely satisfactory.
It is accordingly an object of the present invention to provide a soft toric contact lens with a spherocylindrical power to correct astigmatism. It is a further object of this invention to provide a soft toric contact lens using a prism ballast to maintain axial orientation. It is a yet further object of this invention to provide a soft toric contact lens of improved design for increased user comfort. These and other objects of the invention will be apparent from the ensuing description and claims.